The present invention generally relates to a heating assembly for heating objects and, more particularly, to a heating assembly for an appliance. Most particularly, the present invention relates to a heating plate assembly for a cooking appliance that uses a thin film to supply heat to food items.
A conventional cooktop appliance is taught in U.S. Pat. No. 4,740,664 to Payne et al., which provides a heating assembly that comprises a metal pan, a fibrous insulation lining, a ring, a heating element, and a cooking surface.
In such a conventional assembly, the metal pan, which mounts the assembly to the cooking appliance, must be substantial enough to encase the various parts and to direct the heat generated by the heating element toward a food item that is being heated. Since the metal pan is thermally conductive, the fibrous insulation lining and ring are required to minimize the heat that might be radiated away from the food item, so as to improve the efficiency of the thermal transfer within the metal pan. However, even with the lining, this form of conventional heating assembly tends to lose heat through the lining.
In addition, the heating element, which is typically shaped in a serpentine pattern, is positioned on the surface of the fibrous insulation away from the cooking surface, which acts to transfer heat to the food item. In effect, this construction prevents direct contact between the heating element and the food item. Since the heating element can be somewhat away from the cooking surface, the heat is not efficiently transferred to the item to be heated. Typically in low temperature cooking, the heating element does not “glow” and hence has poor radiant heat transfer characteristics.
Recently, conductive metal oxide thin films have been disposed on glass and employed in place of conventional heating elements, in a variety of applications, where the films are deposited on a substrate, for example, glass, ceramic, or glass-ceramic, preferably by screen printing, spraying, or chemical vapor deposition (CVD).
U.S. Pat. No. 5,932,128 to Dishop teaches deposition of a thin film to a surface of a substrate that is opposite the actual cooking surface. However, with such a construction, the substrate can become capacitively coupled to a metal cooking container and ionic conductivity through the substrate occurs. Because these effects can cause electrical shock and supply ignition for food fires, Dishop provides an elaborate switching control system to minimize the chance for an occurrence of these effects. This control system, however, adds cost to the heating assembly and does not completely eliminate a potential for electrical shock or eliminate ignition for food fires, if the control system fails.
Typically, warming devices operate at moderated temperatures below the boiling point of water, where gentle warming of clothing, melting butter, or preparing sauces would be examples. Silica glass would be suitable as the substrate for disposing the thin films on warming devices, while conventional electronic solders would be appropriate for connecting the various electrical parts together.
Heating devices, on the other hand, require higher operating temperatures, for example, temperatures at or above the boiling point of water, which are required in the case of true cooking. Heating devices may require special construction materials like ceramic or porcelain steel, where, possibly, ceramic-silver frit bus bars and high temperature solders would be required. However, this construction tends to be costly.
Thus, those skilled in the art continue to seek a solution to the problem of how to provide a better heating assembly for a cooking appliance.